The top five players on the men's professional tennis tour are all over 30 years old. 35-year-old Roger Federer won the Australian Open and Wimbledon this year. 31-year-old Rafael Nadal triumphed at the French Open. On the women's tour, 30-somethings like Serena and Venus Williams continue to tear through tournaments.

It's easy to forget that professional tennis used to be famous for young "burnouts."

The arrival of big money in the sport in the 1970s sparked an early-training revolution, with kids barely big enough to hold racquets being drilled for hours every day. By the early 1980s, the first generation of this new era had begun to rebel. Andrea Jaeger, a French Open finalist at 17, hated the extreme competitiveness. She quit before she was old enough to legally drink. "I was labeled 'the brat,' yet in reality I was the one suffering," she said years later.

11-time major champion Bjorn Borg walked away from tennis at just 25, longing for a "normal" life. His rival John McEnroe took a sabbatical from the tour at the same age after dominating the 1984 season. He wrote in his new memoir that he "felt a whole lot emptier than I ever would've thought possible, given the incredible success I'd had." He never made it back to the top of the game.

Tennis burnout carried on into the next decade, with 4-time major champion Jim Courier so emotionally spent that he once read a novel during the changeovers of a match. Teen star Jennifer Capriati spiraled into drug use and said she thought about suicide before making a successful comeback. All-time great Andre Agassi famously declared that during his early years on the tour he "hated tennis."

That ugly narrative started to change about a decade ago, thanks to the so-called "Big 4." Federer, Nadal, Andy Murray and Novak Djokovic seem to find a contentment and joy in playing top-level tennis that hasn't been seen since France's Four Musketeers ruled the courts in the 1920s. And that joie de vivre has been infectious. For years, tennis careers routinely came to a close by the age of 30. Now it's become commonplace to see players out there competing -- and winning -- deep into their fourth decade on the planet.

But this moment in tennis history is often called a "golden age" for a reason. Aging, happy warriors might not become the norm.

After his early loss at Wimbledon this month, talented 24-year-old Australian Bernard Tomic admitted that playing tennis "bored" him, even on SW19's hallowed Centre Court. The comment earned him public censure and a fine from the sport's poohbahs, but he doesn't care. This past week he doubled down on his troubling viewpoint.

"Tennis chose me," Tomic said in an interview for Australian TV. "It's something I never fell in love with. Throughout my career I've given 100 percent. I've given also 30 percent. But if you balance it out, I think all my career's been around 50 percent. I haven't really tried, and still achieved all this. So it's just amazing what I've done."

So Tomic is satisfied with topping out at number 17 in the rankings and occasionally winning a few rounds in the sport's major tournaments.

Fellow Australian Nick Kyrgios, a 22-year-old who's arguably the most talented player of his generation, has said very similar things. "I don't really like the sport of tennis that much," he admitted in 2015. "I don't love it. It was crazy when I was 14. I was all for basketball, and I made the decision to play tennis. I got pushed by my parents, and to this day I can still say I don't love the sport."

Like Tomic, Kyrgios has underachieved and often been accused of lacking motivation and even tanking matches. They appear to be classic burnout cases.

Australian tennis legend Pat Rafter, a top player in the 1990s, says he "couldn't relate" to Tomic and Krygios. He once called Tomic's performance in a 2012 U.S. Open match, when the young player seemed to give up, "disgraceful."

Tomic and Kyrgios can be annoying, with their cocky pronouncements and apparent sense of entitlement. "Disgraceful" often feels like the right word to describe their "who cares?" attitude on court when hundreds or thousands of people have paid good money to watch them play.

But maybe it's a generational thing. Millennials supposedly aren't down with giving over their entire lives to a career, which is what's required for success in pro tennis. They are "the most dissatisfied" with work culture, an Ernst & Young report concluded in 2015, and are "most likely in the survey to say that they would take a pay cut, forgo a promotion or be willing to move to manage work-life demands better."

There's certainly nothing wrong with keeping things in perspective. John McEnroe felt empty after securing the Wimbledon and the U.S. Open titles in 1984 because it was beginning to dawn on him that winning tennis tournaments wasn't the secret to happiness. Tomic and Kyrgios appear to have reached that conclusion without having to win major tournaments. That could be called progress. They've done the professional tennis version of taking a pay cut and forgoing promotion to manage their work-life demands.

Let's hope Tomic, Kyrgios and other talented young players find a middle ground they can live with. Right now Tomic thinks it's "amazing" what he's accomplished considering his discontent -- winning three small, little-known professional tournaments in his career so far and cracking the Top 20. But when he's Roger Federer's age and is in the stands with a beer belly rather than still playing on Centre Court, he might regret having let his athletic opportunities pass him by in favor of other pursuits.